A water absorbent resin obtained by polymerizing an unsaturated monomer containing a carboxyl group is widely used for various applications such as application as sanitary materials, e.g., disposable diapers for babies, sanitary napkins, and incontinent pads, application in agriculture/horticulture, application as a cable waterproofing agent, application in civil engineering and construction, and food related application. The water absorbent resin is typically obtained as a result of processes including drying of a cross-linked polymer hydrogel crushed to a size easy to dry, a pulverization step, a classification step, a surface cross-linking step, and the like. The cross-linked polymer hydrogel is obtained by polymerizing an unsaturated monomer containing a carboxyl group in the presence of an internal cross-linking agent.
In the production of the water absorbent resin, a method used as a method for crushing the cross-linked polymer hydrogel is one or a combination of a method of concurrently polymerizing and crushing in a reaction container including crushing means and a method of crushing the cross-linked polymer hydrogel by a crusher or the like after polymerization without crushing.
In particular, according to the method of concurrently polymerizing and crushing in a reaction container including crushing means, an additional crushing process of the cross-linked polymer hydrogel is not required after polymerization. The cross-linked polymer hydrogel can be dried directly after the polymerization. Therefore, this method is superior in regard to efficiency.
In this way, a fragmented cross-linked polymer hydrogel can be obtained by using the method of concurrently polymerizing and crushing. The fragmented cross-linked polymer hydrogel is different from a clump of jelly-like cross-linked polymer hydrogel obtained by using the method of polymerizing without crushing.
However, recently, in view of water absorbent properties of a water absorbent resin, there is an increasing trend in which polymerization is carried out in the presence of a large amount of internal cross-linking agent. The cross-linked polymer hydrogel is obtained as a hard gel in the presence of a large amount of internal cross-linking agent. Accordingly, when polymerization in the presence of a large amount of internal cross-linking agent is carried out by using the method of concurrently polymerizing and crushing in a reaction container including crushing means, the cross-linked polymer hydrogel cannot be fragmented as finely as in a case where the cross-linked polymer hydrogel is a soft gel. This results in a large piece of gel of the cross-linked polymer hydrogel. This causes a problem such that the large piece of gel remains as an undried cross-linked polymer hydrogel (hereinafter in the specification, the “undried cross-linked polymer hydrogel” may be referred to as an “undried product”) in a case where the cross-linked polymer hydrogel obtained is directly dried.
Because the undried product is like rubber and has very strong cohesion, the undried product has a difficulty in processing. For example, sticking to a pulverizer in a pluverization process, the undried product tends to cause a trouble to stop operation of the pulverizer. Therefore, it is necessary to reduce the undried product as much as possible.
The foregoing disadvantage of the undried polymer occurs not only in case of concurrently polymerizing and crushing the cross-linked polymer hydrogel in the reaction container including crushing means but also in a case of adopting a general method. As a method for solving the disadvantage without excessive drying, there is known a method of separating and removing the particles of the undried product whose volume and mass is larger than particles of a dried product by classification or the like and further recycling the undried product.
The problem above is significant particularly in the case of polymerization in the presence of a large amount of internal cross-linking agent. There are some reports concerning a method of using, as an internal cross-linking agent, a combination, in a predetermined proportion, of (i) an internal cross-linking agent having at least two radical polymerizable unsaturated groups in its single molecule and (ii) an internal cross-linking agent having in its single molecule at least two functional groups that can react with a carboxyl group (Refer to, for example, Patent Documents 1 through 6).
For example, Patent Document 1 discloses a technique in which (i) a compound having at least two unsaturated groups in its single molecule and (ii) a compound having at least two epoxy groups in its single molecule as functional groups that can react with a carboxylic group are used in a specific proportion in polymerization, for the purpose of producing a water absorbent resin that has excellent absorbency against pressure and low extractable polymer content.
Moreover, Patent Document 2 discloses a technique in which a cross-linking agent containing a polyvinyl first cross-linking agent and a hydroxyl-containing second cross-linking agent are used in polymerization for the purpose of producing a water absorbent resin having a high gel bed permeability (i.e. liquid permeability) and a low absorption capacity.
Further, Patent Document 3 discloses a technique in which a compound having at least two unsaturated groups in its single molecule and alkylene carbonate are used in polymerization for the purpose of providing a water absorbent resin generating sound in water-absorption.
In addition, Patent Documents 4 through 6 disclose a technique in which an acrylic ammonium salt, an internal cross-linking agent having at least two unsaturated groups in its single molecule, and a condensation-type cross-linking agent such as glycerin are used for the purpose of improving absorbing performance.    [Patent Document 1] Japanese Unexamined Patent Publication No. 188602/1996 (Tokukaihei 8-188602) (published on Jul. 23, 1996)    [Patent Document 2] Japanese Unexamined Patent Publication No. 518150/2003 (Tokukai 2003-518150) (published on Jun. 3, 2003)    [Patent Document 3] Japanese Unexamined Patent Publication No. 200168/1994 (Tokukaihei 6-200168) (published on Jul. 19, 1994)    [Patent Document 4] Japanese Unexamined Patent Publication No. 200630/2005 (Tokukai 2005-200630) (published on Jul. 28, 2005)    [Patent Document 5] Japanese Unexamined Patent Publication No. 199805/2006 (Tokukai 2006-199805) (published on Aug. 3, 2006)    [Patent Document 6] Japanese Unexamined Patent Publication No. 225455/2006 (Tokukai 2006-225455) (published on Aug. 31, 2006)